The present invention concerns a method of producing microfibrillated cellulose (MFC). The invention even comprises uses of MFC obtained by said method. In connection with the invention “microfibrillated cellulose” or “MFC” also covers what is known as nanofibrillated cellulose (NFC).
Microfibrillated cellulose (MFC) is hereby defined as fibrous material comprised of cellulosic fibrils, which are very thin, of a diameter of about 5 to 100 nm, in average about 20 nm, and have a fibril length of about 20 nm to 200 μm although usually of 100 nm to 100 μm. Nanofibrillated cellulose (NFC) is a specific class of MFC with fibre dimensions at the low end of said fibril size range. In the MFC individual microfibrils are partly or totally detached from each other. Fibers that have been fibrillated and which have microfibrils on the surface and microfibrils that are separated and located in a water phase of a slurry are included in the definition MFC. MFC has a very large open active surface area, generally in the range of about 1 to 300 m2/g, and is useful for a wide range of end used, notably in the field of papermaking.
Prior art methods of manufacturing MFC include mechanical disintegration by refining, milling, beating and homogenizing, and refining e.g. by an extruder. These mechanical measures may be enhanced by chemical or chemoenzymatic treatments as a preliminary step.
U.S. Pat. No. 4,341,807 describes production of MFC by passing a fibrous suspension repeatedly through a small diameter orifice subjecting the liquid suspension to a pressure drop. The starting suspension contains 0.5 to 10 wt-% of cellulose. The product is a homogenous gel-formed suspension of MFC.
WO 2007/091942 A1 describes a process, in which chemical pulp is first refined, then treated with one or more wood degrading enzymes, and finally homogenized to produce MFC as the final product. The consistency of the pulp is taught to be preferably from 0.4 to 10%. The advantage is said to be avoidance of clogging in the high-pressure fluidizer or homogenizer.
The above two prior art references relate to what may be referred to as low consistency (LC) refining through use of dilute suspensions. WO 2012/097446 A1 instead describes a process of making NFC by multipass high consistency (HC) refining of chemical or mechanical fibers. In the reference HC refers to a discharge consistency of more than 20 wt-%. The product comprises a population of free filaments and filaments bound to the fiber core from which they were produced. The fiber diameter is reduced from about 8-45 μm of common papermaking fibers to the nanometer scale of less than 100 nm. The advantage is obtaining NFC with a high aspect ratio, i.e. the length of the fibril in relation to its diameter, which yields improved intrinsic strength of the product.
WO 2012/072874 A1 teaches a multistep process of producing NFC, in which cellulose is refined with a first refiner, the product is divided into an accept fraction and reject fraction, water is removed from the accept fraction, and finally the accept fraction is refined with a second refiner to obtain a gel-like product with fiber diameter of 2 to 200 nm. At the first refining step the consistency of the material is under 10% but increased by removal of water to about 15% or even 20% to enhance washing of the same. For the second refining the pulp would be diluted back to a consistency under 10%.
U.S. Pat. No. 3,382,140 teaches preparation of high consistency papermaking pulp, in which pulp obtained from kraft pulping is first dewatered to obtain a non-fluid or semi-solid mass, which is non-pumpable. The dewatered mass, having a consistency of 20 to 30%, is then refined, diluted to a conventional papermaking consistency and formed into a paper web. There is no mention of production of MFC, and the teaching of direct use of the mass as a papermaking furnish does not suggest MFC either, the latter being known as a minor additive in such furnishes only.
In WO 2011/114004 there is described a different approach of fibrillating lignocellulosic material based on treatment with ionic liquid, i.e. molten salt, which preserves fibres basically intact. Salts comprising an imidazolium type cation are mentioned as an example of such liquids. The process is said to weaken the binding between fibrils or tracheids and separate fibrils or tracheids from fibre walls.